The carrier flies to the Croatian capital Zagreb from Heathrow and owns nine weekly slot pairs.

It is thought that the carrier is unable to compete profitably with British Airways at Heathrow, while Monarch will launch flights to Zagreb from Gatwick in April – making a move to Stansted more likely for Croatia Airlines.

Slots at congested Heathrow are commanding record prices, with one prized early morning slot recently fetching $75 million when Kenya Airways sold it to Oman Air. Croatia Airlines’ slots are mid-morning and early evening, so will be less valuable.

If Croatia Airlines quits Heathrow, it will join a growing line of carriers that have cashed in and left, including Flybe, Lux Air and, before it collapsed, Cyprus Airways.

Airlines including major carriers like British Airways and Virgin Atlantic are becoming increasingly vocal about the cost of operating from Heathrow, with the subject likely to be a hot topic as the debate over expanding the airport comes to a head.